Stent prostheses are known for implantation within body lumens to provide artificial radial support to a collapsing, weakened, and/or occluded passageway, such as blood vessels of the body. Stent prostheses are typically constructed of a metal or polymer and are generally a hollow cylindrical shape. When a balloon-expandable stent is to be implanted, a balloon catheter carrying the stent mounted on its balloon is advanced to the target site, such as a stenosis. The balloon and accompanying stent are positioned at the location of the stenosis, and the balloon is inflated to circumferentially expand and thereby implant the stent. As the balloon expands, it physically forces the stent to radially expand such that the outside surface of the stent comes into contact with the vessel wall. Thereafter, the balloon is deflated and the balloon catheter is withdrawn from the patient, leaving the stent in the expanded or deployed configuration.
One criterion for successful stent deployment is apposition of the stent against the vessel wall, since any regions of the stent that protrude into the lumen may cause blood turbulence, which in turn may lead to acute thrombosis and arterial blockage. By “apposition” or “wall apposition” herein it is meant that at least the outer surface of the deployed stent is fully positioned against, i.e., makes contact with, the vessel wall. The desire to prevent incomplete apposition leads to the common practice of dilating the stent after it is initially expanded with a second high-pressure balloon. This procedure intentionally overexpands the stent to ensure full apposition but may cause unnecessary injury to the vessel, which may lead to the primary failure mode of stents: the chronic hyperproliferative growth of smooth muscle cells called neointimal hyperplasia. This growth impinges on the lumen and reduces blood flow, which in approximately 30% of all stented patients is severe enough to require further intervention within three months of stent implantation.
In addition, the geometry or shape of many vessel lesions compounds the problem of incomplete vessel apposition. A so-called focal lesion is one that has a generally thicker midsection with one or more tapered ends, with the thicker midsection resulting in a constricted or occluded body lumen. When a stent is expanded over a focal lesion, the proximal and distal ends of the stent may not be apposed to the vessel wall although the middle of the stent has expanded sufficiently to restore patency. In order to achieve full apposition, the proximal and distal ends of the stent may require further expansion relative to the middle of the stent.
There exists a need in the art to ensure that a balloon expandable stent achieves complete or full apposition against the vessel wall during deployment. It is an object hereof to provide a stent delivery system that enables a physician the ability to adequately deploy a stent such that the implant will have complete apposition against the vessel wall.